Glass-ceramic materials comprise an amorphous (glass) phase and one or more crystalline (ceramic) phases embedded in the amorphous phase. Due to the presence of both an amorphous and a crystalline phase, glass-ceramics share many properties with both glasses and ceramics. They are used in a variety of different technical fields, for example as cooktops, cookware and bakeware, as a substrate for magnetic disks or as high performance reflectors for digital projectors.
Glass-ceramics are of particular interest in the field of restorative dentistry, in which the need for prostheses that, in terms of functionality and appearance, would perform exactly like their natural counterparts has been expressed.
Conventionally, dental restorations have been prepared according to the “porcelain fused to metal” (PFM) method in which the supporting metal framework is used in conjunction with a veneering layer of a ceramic material which makes up the colour of the prosthesis. The preparation of restorations according to this method implies many manufacturing steps and is hence laborious.
The PFM method has been developed further by replacing the metallic framework by a non-metallic inorganic framework. In this regard, a feldspathic glass filled with alumina particles has been proposed. Further development has led to substituting an opaque ceramic framework for the alumina-reinforced glass.
Dental crowns and bridges are today mostly manufactured by CAD/CAM technologies, which are increasingly gaining importance. The fabrication process comprises two decisive stages: a computer-aided design of the restoration and its computer-aided milling. In the stage of milling, the restoration is machined out of a blank.
DE-A-19750794 has proposed a process for preparing a lithium disilicate glass product suitable for the use as a dental product. The process is aiming at a high chemical stability, a high translucency and good mechanical properties of the product. Due to the high strength and toughness obtained, the machining of the material results, however, in a very high wear of the machining tools and very long processing times. Furthermore, restorations prepared according to this technique show only a poor strength when their thickness falls within a range of only a few hundreds of micrometers.
U.S. Pat. No. 7,452,836 relates to a process for providing a glass-ceramic which has metastable lithium metasilicate (Li2SiO3) as main crystalline phase. This lithium metasilicate glass-ceramic has mechanical properties allowing it to be easily machined into the shape of even complicated dental restorations without undue wear of tools. It can be converted by further heat treatment into a lithium disilicate glass-ceramic with very good mechanical properties and translucency.
Although U.S. Pat. No. 7,452,836 allows for achieving materials having a flexural strength which might be sufficient for the restoration of multiple missing teeth in the front (for example 3-unit bridges), its strength is still not sufficient for posterior bridges or large restorations.
Aiming not only at an improvement in mechanical properties, but also at a highly aesthetical appearance, a material having an internal structure mimicking the structure of a natural tooth would be highly appreciated.
Natural teeth consist of a hard, inert and acellular enamel supported by the less mineralized, more resilient and vital hard tissue dentin. Because of its exceptionally high mineral content, enamel is a brittle tissue unable to withstand the forces of mastication without fracture unless it has the support of the more resilient dentin.
Enamel and dentin do not only differ in their mechanical properties, namely their compressive strength, elastic modulus and coefficient of thermal expansion, but also in their appearance. Whereas enamel is translucent and varies in colour from light yellow to gray-white, dentin is yellow. In a natural tooth, the thickness of enamel varies from a maximum of approximately 2.5 mm to a fraction thereof. This variation influences the tooth's appearance because the underlying dentin is seen through the thinner enamel region, whereas it gradually fades out towards thicker ones.
In summary, a natural tooth has thus an inhomogeneous structure different than in the glass-ceramic of U.S. Pat. No. 7,452,835, in which crystals are grown throughout the whole volume without any spatial order. In contrast to a natural tooth, which exhibits a different composition and structure in different parts, be it in the dentin or the enamel part, a restoration made of the material according to U.S. Pat. No. 7,452,835 is with respect to the material constitution rather homogeneous and does not comprise regions of different constitutions like the natural counterpart does. A natural tooth can thus not be mimicked perfectly by the material according to U.S. Pat. No. 7,452,835.
Biocompatible, highly aesthetical and robust materials with an internal structure mimicking that of a natural tooth for a single tooth replacement (crowns) and for a prosthesis formed by two or more crowns (bridges) supported by modified natural teeth are, however, of paramount importance in the field of restorative dentistry. Further, as more dental laboratories adopt CAD/CAM devices, laboratory-generated CAD/CAM prostheses are expected to rise significantly in the decades ahead. This evolution poses an additional requirement to materials for the fabrication of restoration viz. CAD/CAM machining at affordable costs.
A method for manufacturing prostheses from a blank comprising at least one layer of high abrasive resistance, at least one layer of high flexural strength and at least one layer of lower hardness and strength is disclosed in U.S. Pat. No. 5,939,211. During the milling of the restoration, material removal is performed in such a manner that layers with high strength constitute a reinforcing structure.
Based on the finding that a so-called functionally graded material can lead to an improved resistance in contact damages, US 2008/0213727 proposes a process for providing a functionally graded material including infiltrating top and bottom ceramic surfaces with glass. The resulting structure comprises an outer (aesthetic) surface residual glass layer, a graded glass-ceramic layer and a dense interior ceramic.
Further, WO 2010/010082 aims at a material mimicking the colour gradients in a natural tooth and relates to a form-stabilized material comprising a first component and a second component, the second component having a different pigmentation than the first component and being disposed in the first component such that the boundary surface between the components represents a spatially curved surface.
In particular regarding U.S. Pat. No. 5,939,211 and WO 2010/010082, the presence of physically distinct component layers and thus of an interface between different components can have an impact on the overall stability of the dental restoration. Also, the processes according to these documents are relatively laborious.
The technique according to US 2008/0213727 allows a gradient of only a very small thickness to be formed. In addition, the gradient is confined to the surface area of the material; the formation of a gradient within the bulk of the material remote from the surface is however not possible according to US 2008/0213727.